This invention relates to an active Power Factor Correction (xe2x80x9cPFCxe2x80x9d) circuit. More particularly, the present invention describes several methods and an apparatus for supporting a Parallel Charge, Series discharge (xe2x80x9cPCSDxe2x80x9d) system, to be used in a Power Factor Correction process between a generator and a load, which includes reactive components.
In the process of transferring AC electrical power from a generator to a load, in addition to the efficiency, the power factor is a very important parameter. The Efficiency (xcex7) is technically the ratio of Output Power (measured in Watts) to the consumed Input Power (measured also in Watts) and often expressed as a percentage (0 to 100%).
xcex7(%)=100xc3x97Output Power (W)/Input Power (W); 0% less than xcex7 less than 100%
This above expression illustrates that the Efficiency (xcex7) of an electrical system is determined only by the parameters of Output Power and Input Power. The ideal efficiency is 100%, meaning there is absolutely no loss in the generator-load circuit. In real AC/DC converter situations, the efficiency is often as low as 60%. In the most modem designs, by using new system architectures and suitable parts, converters with efficiencies of greater than 95% are possible.
Power Factor (xe2x80x9cPFxe2x80x9d) is traditionally known as the cosine of the phase difference between a sinusoidal source voltage and the corresponding load current waveform. In fact, this thinking is only valid when the input current waveform is sinusoidal. For the general case and valid for all situations, PF is the ratio of Real Input Power consumed to Apparent Input Power and is expressed as a decimal fraction between 0 and 1. The Real Power (in some other publications may be called xe2x80x9cTrue Powerxe2x80x9d, xe2x80x9cAverage Powerxe2x80x9d or xe2x80x9cRAM Watts (sic) Powerxe2x80x9d) is measured in Watts (W) and the amount is equal to the time integral of the input voltage and input current product. The Apparent Power is measured in Volt-Amperes (VA) and the amount is equal to the product of the rms input voltage and the rms input current.
PF=Real Input Power (W)/Apparent Input Power (VA); 0 less than PF (W/VA) less than 1.
This above expression shows that the PF of an electrical system refers only to characteristics of the Input Power. Since the Output Power is not involved in the PF expression, a low Power Factor is not necessarily related to low efficiency, but rather with a low utilization of the Apparent Input Power. The ideal value for the Power Factor is PF=1, which means that the Real Input Power is equal in amount to the Apparent Input Power. This happens only when the entire load circuit is purely resistive, because resistors do not change the current phase or current shape from the source voltage. For example, for a simple AC generator-resistive load circuit (FIG. 1A), the shape and phase of the current waveform of the generator""s circuit (FIG. 5B) is identical to the shape and phase of the voltage waveform generated into the circuit (FIG. 5A), assuming the voltage drop of the bridge rectifier is negligible. For this case the Real Input Power is equal to the Apparent Input Power, so PF=1.
However, when the load is non-linear, time varying, or contains reactive elements (e.g., capacitors and inductors), the current waveform in the generator""s circuit becomes very different in shape and/or phase than the voltage waveform and PF decreases. In the xe2x80x9cbridge rectifier-bulk capacitorxe2x80x9d case (FIG. 1B), the capacitor Cload acts a storage device (exactly the desired filtering function), keeping the DC voltage across the load close to the peak of the rectified input voltage. This means that the capacitor charges only for a small part of the AC cycle, i.e., the capacitor only charges during the time when the AC peak voltage (minus the small voltage lost on the bridge rectifier) exceeds the instantaneous capacitor voltage. The capacitor stops charging just as the AC voltage reaches its maximum peak value, because after that moment, there is no more current through the rectifier bridge (as long the load""s voltage is higher than the generator""s voltage, BR""s diodes are reverse-biased). This results in a current pulse lasting typically for only 1 or 2 mS of the 8.33 mS half cycle (based on a 60 Hz source). This pulse waveform (see FIG. 5C) is a dramatically different shape from the incoming sine wave voltage and many harmonics are created in the current""s waveform spectrum. Since only the fundamental 60 Hz component of the current can contribute to the Real Power, the current from each harmonic increases the Apparent Power amount. This also increases THD (Total Harmonic Distortion), typically to over 100%, and decreases the PF down to typically less than 0.65. Thus, the rms input current is higher than otherwise necessary, so the electrical utilities need to have more generating and distributing capacity. In addition, because the impedance of the power lines and distribution transformers is not zero, the harmonic currents distort the voltage waveform and can cause problems with other equipment.
Virtually all existing electrical devices supplied from AC power sources have Power Factors less than 1.0 (with the obvious exception of pure resistive loads, such as heating devices and incandescent lamps). Because of the problems that low PF creates, PF correction is desirable for a wide range of electrical devices. However, the extra cost and complexity of the PF circuit have to be weighed against the advantages of improved PF. Also, in a given system, by attaching a PF circuit, the overall efficiency of the entire system will almost always decrease due to the less than 100% efficiency of the PF circuit itself. (Exceptions exist for cases where the addition of PF Correction enables improved efficiency in the rest of the system, by increasing the internal DC operating voltage or by other effects.) Therefore, PF, Harmonic Distortion, cost and efficiency are the most important parameters of a PFC circuit.
Presently, worldwide, the number of electrical devices containing internal electronic circuits (such as computers, TV sets and computer monitors, stereos, industrial equipment, telecommunication equipment, etc.) is increasing dramatically every day. These devices require an internal DC voltage supply, obtained by converting the AC current available from the AC power line. Often a simple circuit like the one illustrated in FIG. 1B is used for this AC to DC conversion. Therefore, there is a worldwide need to resolve low PF issues efficiently and cost effectively. As an example of the seriousness of the problems caused by low PF equipment, Japan and the European Union have set standards for PF and Harmonic Distortion. These standards cover a wide variety of electrical devices.
Although there are methods to increase PF and lower THD using passive components (inductors and capacitors), generally the size and cost of the passive components is prohibitive for most common electronic equipment. Considerable effort has been expended over the last 15 years to develop so-called active methods of power factor correction.
FIG. 1C illustrates the core of most classic active PFC systems and contains an inductor, L, a switching diode, D, and an electronic switch, SW. Four terminals, Vin0, Vin1, Vout0, and Vout1 connect this block to the external circuit. Commonly, an AC voltage generator, Gac, provides energy to the system through a Low Pass Filter, LPF, and a bridge rectifier, BR. (The purpose of LPF is to reduce substantially the amount of electrical high frequency noise generated in the PFC circuit that appears as high frequency currents in Gac.) The unfiltered full-wave rectified output from BR appears with the positive pole at Vin1 with respect to Vin0. A continuous series of high frequency pulses is delivered through D to the reactive load, Zload, that includes a resistive load and a large value bulk storage capacitor. The polarity is positive at Vout1 with respect to Vout0. The electronic switch SW (usually a power MOSFET) switches OFF and ON at a relatively high frequency. While SW is ON there is a path for current from BR through L. The current in L increases until SW is OFF. At this point, some (if not all) of the energy stored in L will be delivered to Zload in a discharge circuit which includes L, D and Cf (BR and LPF also include a part of -the load current in parallel to Cf). The ON and OFF times of SW, together with an appropriate value for L, can be arranged to force the input current waveform shape (FIG. 5D) to be very close to the voltage waveform (FIG. 5A), despite the large value capacitor included in Zload.
FIG. 1D illustrates the core of a classic PFC circuit schematic diagram, which provides more circuit details. A common active PFC circuit contains an inductor, L, a switching diode, D, a power MOSFET, Q, a switching controller, SC, a current sense resistor, Rs, and a voltage sense divider, R1 and R2. The switching controller, SC, provides the gate of MOSFET Q, with a PWM (Pulse Width Modulated) signal with proper ON-OFF times. This gate signal must be properly related to the instantaneous amplitude of the input voltage at Vin1 (with respect to Vin0) and with L and ZL currents. Therefore, SC must have at least a current sense input, Is, and sometimes a voltage sense input, Vs. Usually SC also has an input, Vos, for use in regulating the output voltage and some other inputs (not specified here). A classic PFC controller IC has, typically, 8 to 16 terminals.
Similar to FIG. 1C, FIG. 1D classic PFC circuit has four terminals, Vin0, Vin1, Vout0, and Vout1, that connect the PFC block to the external circuit. Commonly, an AC voltage generator, Gac, provides energy to the system through a thermistor, Th, with a first filtering capacitor, Cf1, a filtering inductor, Lf, a second filtering capacitor, Cf2, a bridge rectifier, BR, and a third filtering capacitor, Cf3. The full-wave rectified output from BR appears with the positive pole at Vin1 with respect to Vin0.
A continuous series of high frequency pulses is delivered through D to the reactive load, Zload, which includes a resistive load, Rload, and a large value capacitor, Cload. The polarity is positive at Vout1 with respect to Vout0. Until it is fully charged after first turn-on, the load capacitor, Cload, will absorb a high current from the Gac generator (inrush current). In order to reduce this initial current surge, the thermistor, Th, limits, for a short time, the current in the entire circuit, by having large electrical resistance when it is cold (room temperature) and relatively small electrical resistance at high temperature. Upon first turn-on, Th self-heats due to the initial current and the consequent circuit current and stays hot (low resistance) during normal operation. When Th is hot, the capacitor, Cload, is already fully charged and the input circuit current is basically the average of the inductor, L, current. In order for the generator, Gac, current waveform to follow its sine wave voltage waveform, SC must deliver appropriately timed ON and OFF high frequency pulses to the gate of MOSFET Q. By sensing continuously the input circuit current (sometimes along with the input voltage), the controller SC is able to provide properly timed ON-OFF pulses for a good power factor and low THD. This circuit is able to improve the power factor to over 0.95 with low THD, but despite using a complex (and expensive) controller IC, the efficiency is normally less than 90%. Other disadvantages are a relatively large value of the switching inductor (about 1 mH) which increases the loss of energy and the cost, RFI/EMI problems due to square wave voltages across MOSFET Q, high switching losses in MOSFET Q, and, in some designs, high switching losses in D. High switching losses in Q and D require a cost penalty for large heat sinks and of course, the efficiency is reduced due to the switching losses. An important limitation of this classic circuit (sometimes referred to as a xe2x80x9cboostxe2x80x9d circuit) is that the output DC voltage must be higher than the input peak voltage in order to provide good PF.
Therefore, in a situation where the input rms voltage can be (temporarily) as high as 260V (European nominal 240V), the peak input voltage becomes 260Vxc3x971.414=368V, the output DC voltage must be higher than 370V. Commonly, PFC circuit designers provide a DC output of more than 380V for 240VAC applications.
Therefore, a need exists for a PFC circuit that is cost effective and efficient, does not need a sophisticated switch control circuit, is able to use a smaller inductor for a given output power, does not need a thermistor (or any other protection system) for the inrush current, and is able to provide an output voltage either higher or lower than the peak input voltage, with good PF and low Harmonic Distortion.
Accordingly, the present invention is directed to a Parallel Charge Series Discharge Power Factor Correction (xe2x80x9cPCSD-PFCxe2x80x9d) circuit, including system, methods, and apparatus, that addresses many of the problems due to limitations and disadvantages of the related art.
The internal structure of the PCSD-PFC (10) circuitry may include five principal functional blocks: a first impedance circuit Z1 (1), a second impedance circuit Z2 (2), a first switch S1 (3), a second switch S2 (4), and a third switch S3 (5), as shown in FIG. 2A.
The generic PCSD-PFC (10) invention may include four distinct in/out terminals, and other embodiments, having less or more than four terminals, representing particular variations and are based on the same method and principles. The basic terminals are Vin0 (12), Vin1 (11), Vout0 (13), Vout1 (14). These four in/out terminals may be used separately and/or simultaneously.
It is an object of the present invention to provide a new technique of improving the PF in an electrical circuit, by the means of a Parallel Charge Series Discharge (PCSD) system, which includes at least two devices, e.g. a capacitive and an inductive circuit, able to store energy and charging/discharging switching circuitry.
It is still another object of the present invention to provide several methods of controlling the PCSD system that improve the PF during the transfer of the electrical energy between one (or more) AC electrical generator(s) and one (or more) reactive and/or non-linear load(s) by charging, in a parallel circuit, a fraction of the incoming electrical energy into two (or more) device(s) able to store electrical energy, and then to discharge the two (or more) device(s), coupled in a series circuit this time, into one (or more) reactive load(s).
It is another object of the present invention to provide a new circuit architecture that is efficient, and is able to reduce substantially the dimensions and/or cost of a PFC circuit through the use of a PCSD electronic switching system.
It is also another object of the present invention to provide an xe2x80x9cAC generator-bulk capacitorxe2x80x9d apparatus that not only is a PF corrector but also includes a complete power supply unit, incorporating an internal PCSD system.
To achieve these and other advantages, and in accordance with the purpose of the present invention, as embodied and broadly described herein, the present invention is a Parallel Charge Series Discharge (xe2x80x9cPCSDxe2x80x9d) system, apparatus and method. One embodiment of the PCSD circuit comprises: an electrical power source, a first impedance for charging and storing a fraction of the electrical energy incoming from the electrical power source, a second impedance for charging and storing a fraction of the electrical energy incoming from the same or a different electrical power source, at least one switch having at least a first switch position for charging in a parallel circuit said first and second impedances, and at least a second switch position for discharging in a series circuit said first and second impedances into one or more reactive loads.
The invention exists in the novel parts, constructions, arrangements, combinations and improvements herein shown and described. The above stated and other objects and advantages of the invention will become apparent from the following description when taken with the accompanying drawings. It will be understood, however, that the drawings are for purposes of illustration and are not to be construed as defining the scope or limits of the invention, reference being had for the latter purpose to the claims appended hereto. Additionally, it will be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. In addition, the accompanying drawings illustrate the embodiments of the invention and, separately as well as together with the description, serve to explain the principles of the invention.